Current studies are directed toward elucidation of the mechanisms and structure of the (Na ion plus K ion)-ATPase which is the enzymatic expression of active sodium transport. The two polypeptide subunits have been isolated from the enzyme. Antisera to each have been prepared and employed in studies which indicate that two antigenic loci are well separated. Transient kinetics of phosphorylation show evidence for a new enzyme intermediate which accumulates in the early stages of the ATPase reaction. Steady-state kinetics have shown a pH and temperature dependence of the conformational equilibrium of the ATPase.